PROJECT SUMMARY Transcranial Direct Current Stimulation (tDCS) is investigated to treat a broad range of brain disorders and to change cognition in healthy individuals. The scale and breadth of tDCS human trials has outpaced understanding of cellular mechanisms. The flexibility of tDCS derives from use in combination with a training task, with the goal to enhance ?neuronal capacity? for plasticity (learning) on the specific task. tDCS is thus applied either during or before a task, to produce an acute or persistent change in neural capacity. The rational advancement of tDCS as a clinical/neuroscience tool requires knowing the cellular targets of stimulation, and linking their activation with changes in neuronal capacity during and after tDCS. Neurons, and to a lesser extent glia, have been studied as tDCS cellular targets. Endothelial cells of the blood-brain barrier (BBB) have been unaddressed until recently by our team. Yet BBB function is well known to be sensitive to other forms of electrical stimulation, and that changes in BBB will alter brain function. Indeed BBB stimulation is consistent with the concept of tDCS acting to generally prime the brain (e.g. changing excitability or metabolic capacity). This proposal addresses a novel hypothesis and scientific premise for how BBB modulation may enhance neural capacity during or after tDCS. We propose that the conductive vascular network across the brain shunts current and in the process generates electric fields across the BBB higher than around neurons. We believe that BBB polarization by tDCS alters the transport of water and solutes across the BBB (during stimulation) and activates the expression of genes leading to the production of neuroactive chemicals (including NO) by the blood vessels of the BBB (after stimulation), all of which modulate the microenvironment of neurons and neuronal capacity. Given a natural bias toward interpreting any tDCS actions as reflecting direct neuron activation (and thus BBB response as secondary/epiphenomena) we require state-of-the-art modeling and experimental tools to quantify the direct stimulation of BBB by tDCS. We present substantial preliminary data from in silico, in vitro, and in vivo studies that support our overall premise. This data reflects a successful R21 collaboration by our team; having shown feasibly of a novel cellular target, this RO1 establishes the mechanism and potential impact of direct BBB activation by tDCS. Aim 1: We will develop a multi-scale (from head anatomy to micro-vasculature) multi-physics (coupling electric fields with electro-diffusion filtration transport) model. Aim 2: We will validate acute (during DCS) changes in water and molecule permeability using a specially designed in vitro BBB model system where the absence of neurons establishes a direct action of current on the BBB, as well as test the activation of nitric oxide (NO) and other neuro-active genes (neurotrophins) by DCS in the absence of neurons. Aim 3: Using multi-photon brain imaging for determining BBB permeability in a rat model, we will analyze the persistent (minutes) BBB permeability changes induced by tDCS and their dependence on NO.